task/edit - Edits an existing task.

Method parameters:
id - The ID of the task to edit.
text - The new text of the task.
deadline_time - The new Unix timestamp of the deadline of the task.
tags - A space-separated list of tags.
top_priority - Whether the task should be marked as top priority.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
TASK_NOT_FOUND - A task with the ID doesn't exist.
ENTER_TEXT - The new text is empty.
TOO_MANY_TAGS - More than 5 tags given.